The present invention relates to improvements in power trains for use in motor vehicles, and to improvements in methods of operating such power trains. More particularly, the invention relates to improvements in power trains of the type wherein a prime mover (such as an internal combustion engine or a motor) is arranged to transmit torque to a variable-speed transmission which, in turn, transmits torque to the driven wheels of the motor vehicle, and wherein an automated torque transmission system is installed between the prime mover and the transmission or downstream of the transmission to transmit a variable torque.
The torque transmission system (such as a friction clutch and hereinafter referred to as clutch for short) can be adjusted to transmit zero torque, to transmit a maximum torque, or to transmit a selected torque less than maximum torque. As a rule, the means for selecting the condition of the clutch comprises a signal receiving, processing and transmitting control unit and at least one so-called actuator which is responsive to signals from the control unit and directly or indirectly adjusts (i.e., selects the condition of) the clutch so that the latter can transmit torques of a desired magnitude. The control unit can receive signals from one or more sensors which are designed to monitor certain parameters of the prime mover, of the transmission, of the clutch, and/or of other constituents of the motor vehicle.
Power trains of the above outlined character are disclosed, for example, in commonly owned U.S. Pat. No. 5,176,234 granted Jan. 5, 1993 to Reik et al., in commonly owned U.S. Pat. No. 5,632,706 granted May 27, 1997 to Kremmling et al. for "MOTOR VEHICLE WITH ELECTRONIC CLUTCH MANAGEMENT SYSTEM", and in commonly owned U.S. Pat. No. 5,679,091 granted Oct. 21, 1997 to Salecker et al.
The disclosures of all U.S. patents and/or pending U.S. patent applications and/or foreign patents and/or foreign patent applications which are analyzed, otherwise discussed and/or merely mentioned in the specification of this application are incorporated herein by reference.
As a rule, motor vehicles of the type disclosed in the above enumerated patents comprise a prime mover in the form of a combustion engine or a hybrid system such as a combustion engine and an energy storing device and/or an electric motor. The clutch normally receives torque from a rotary output element of the prime mover (such as a camshaft or a crankshaft of an internal combustion engine), and the operation of such clutch is or can be automated; for example, the clutch can be adjusted (to a fully engaged, partly engaged or fully disengaged condition) by one or more actuators, e.g., by at least one actuator of the type having at least one electric motor or another driving component and a gearing, a fluid-operated system or a mechanical system for controlled transmission of motion from the driving component(s) to an adjustable part of the clutch. The actuator or actuators respond to signals from a normally electronic control unit which processes signals being transmitted thereto by one or more sensors and/or other signal generating means. The transmission can constitute a manually shiftable transmission which can be manipulated by the operator of the motor vehicle and is normally shiftable into neutral, into reverse gear and into any one of several forward gears. However, it is also possible to employ an automated or a fully automatic transmission or an infinitely variable transmission with adjustable pulleys or sheaves and an endless chain or belt which is trained over such adjustable parts. The transmission can operate with or without an interruption of pulling force.
A drawback of many presently known motor vehicles which embody the so-called electronic clutch management systems (known as ECM) is that their operation is not always predictable with a desired or required degree of accuracy. For example, if the vehicle is caused to creep or crawl (this normally involves a longer-lasting operation of a friction clutch with at least some slip between driving and driven parts), the friction linings (as well as certain other parts) of the clutch can become overheated as a result of a prolonged operation with slip, and this can entail a partial or complete destruction of the afflicted clutch.
Another drawback of presently known power trains of the above outlined character is that their operation is not fully predictable or not always predictable when the temperature of the clutch and/or of certain other component or components (such as the engine) of the power train varies within or beyond a certain range. For example, if a cold engine is to be started in a so-called cold start phase, the available engine torque and/or the idling RPM of the engine is higher than when the starting operation involves a heated or hot engine having a relatively low idling RPM. This entails that the motor vehicle is caused to creep or crawl at a higher speed or that the creeping or crawling movement is arrived at within a shorter interval than when the engine was cold prior to starting.
On the other hand, it is highly desirable and advantageous to ensure that the starting of a motor vehicle, as well as the stage or stages when the vehicle is caused or expected to creep, are fully predictable and reproducible with a very high degree of accuracy. This enables the operator of the motor vehicle to invariably operate the vehicle with a high degree of confidence and competence, not only for the purpose of enhancing the safety of but also for the purpose of enhancing the comfort to the occupant(s) of the vehicle. In other words, the operator of the motor vehicle should be in a position to accurately estimate the mode of operation of the vehicle, not only during travel at a permissible speed but also during starting, initial acceleration and/or crawling or creeping movement preceding the initial acceleration. Furthermore, it is equally desirable that the just discussed desirable characteristics of a motor vehicle during starting, during crawling and during initial acceleration remain at least substantially unchanged during the entire useful life of the vehicle, at least as long as the vehicle is adequately serviced at prescribed intervals.
Still further, at least certain presently known motor vehicles exhibit the drawback that the engine RPM decreases to and beneath an undesirably low value while the motor vehicle is caused to creep, i.e., while the engine furnishes an increasing creep torque. This, too, is undesirable under numerous circumstances of use of such conventional motor vehicles.
Other problems develop, or can develop, when a motor vehicle employing a presently known power train with an automated clutch and an engine which cooperates with the clutch and with a transmission to impart to the vehicle a creeping or crawling movement is braked while it performs a crawling movement. For example, it can happen that one or more brakes are being applied by the operator of the motor vehicle in the course of a crawling movement for the purpose of terminating such movement but the braking to a full stop is interrupted before the vehicle actually comes to a halt. Under such circumstances, the movement of the nearly arrested but still crawling vehicle is not sufficiently predictable or is not predictable at all.